Noninvasive optical imaging diagnostics to quantitatively measure absorption and scattering characteristics of biological tissues are a fundamental prerequisite for a growing number of physiological monitoring procedures and preventative protocols, such as cerebrovascular oxygenation status, photodynamic dosimetry, transillumination shadowgraphy of malignant lesions, and dermatological and ocular photopathology. Although state-of-the art optical imaging performance must be substantially improved to achieve the resolution currently obtainable with conventional radiological modalities such as x-ray computed tomography (CT), positron emission tomography (PET), and magnetic resonance imaging (MRI), optical diagnostics offer unique promise of significantly lower cost and reduced instrumental complexity in acute care environments. Additionally, probing the near-infrared spectral region (600-1300 nm), or so-called "therapeutic window," is a potentially useful noninvasive paradigm for examining internal tissue histopathology owing to the relatively high penetration depths available at these wavelengths with negligible attendant risk of collateral damage from ionizing radiation.
The near-infrared is the spectral region of greatest optical transparency, but multiple scattering is pervasive in fatty tissue at these wavelengths and hinders image formation and the determination of absorption coefficients. Elastic scattering inhomogeneities originating from cellular components (chromatin, organelles, membrane interfaces, intracellular fluid, solutes such as glucose, etc.) with dimensions on the order of an optical wavelength modify the bulk index of refraction (n=1.35-1.5) and cause strong random diffusion of light. Significant temporal and spatial dispersion is induced by this random scattering process on optical pulses, and results in a broad distribution of scattering trajectories and associated pathlengths for radiation traversing the medium.
The nature of transmitted or reflected light ranges from the quasi-coherent properties of the minimally scattered component to the random incoherent light of the diffuse component. Light propagating in tissue will typically lose coherence memory after traveling only a few millimeters, as each photon on average undergoes 100-1000 scattering events with an increase in effective pathlength of over tenfold. Diffuse scatter is the main obstacle to quantitative implementation of optical imagery and tomography in turbid environments.
As opposed to conventional radiological methods, in which most x-ray photon trajectories are along the instrumental line-of-sight, optical wavelengths undergo multiple scattering. The result is a broad distribution of optical propagation delays and irregular pathlengths that degrade (or blur) underlying images and progressively scramble the geometric correlation between incident and detected light. The resulting pathlength uncertainty associated with the detected signal adversely effects the quantification of absorbance and the accuracy of range-resolved optical sensing measurements derived as a function of propagation delay time. In addition, the diffuse background level reduces the effective dynamic range and maximum penetration depths available for unambiguous object detection. Compensation for these "fog-like" effects requires dynamic path-sensitive imaging methods which rely on spatial, temporal, and phase signatures to actively discriminate against diffusely scattered light background.
Theoretically, the dielectric function (phase and amplitude image) may be reconstructed from the diffuse emission if the spatial and temporal distribution of the phase and amplitude are known simultaneously at many discrete points. However, the uncertainty of path of the detected signal makes the inverse scattering problem inherently underdetermined in the absence of deterministic constraints (restrictive assumptions) or simple boundary conditions. Computational approaches to model photon fluence propagation in highly scattering media based on the diffusion approximation to radiative transport theory and numerically intensive Monte Carlo particle-in-cell simulations have been similarly difficult to implement for realistic scenarios; i.e., a stochastic boundary condition imposed by a random interface.
Tradeoffs exist between the temporal and phase-resolved frequency domain approaches to optical imaging in multiple scattering media. The main advantage of frequency domain measurements with modulated light are the detection of the majority of the re-emitted light (diffuse) in the form of photon density waves over large penetration depths. These low-frequency evanescent waves, in contrast with electromagnetic waves, are solutions to the photon diffusion equation and are characterized by a phase velocity and modulation wavelength which are primarily functions of the optical diffusivity. However, the diffuse imaging approach relies on having a spectroscopic model for interpretation, and the diffuse character of the measurement variable makes achieving adequate spatial resolution in the image potentially problematic without the use of phased array sources in an interferometric geometry.
The basic strength of time-resolved spectroscopy is that no mathematical manipulation is needed to interpret raw data, as the photon signal reveals the full time-domain distribution of photon travel (time-of-flight) and explicitly determines scattering and absorption properties. In the time domain approach, pathlength is explicitly determined for each detected photon rather than an ensemble average, resulting in good spatial localization at the expense of the small signal size associated with measuring only the coherent component. Additionally, all-optical time domain techniques allow examination of sampling timescales corresponding to gigahertz frequencies inaccessible by current frequency domain techniques, albeit at the expense of the instrumental complexity associated with ultrashort optical technology. However, the instrumentally challenging aspects of short pulse generation and its associated expense has been considerably reduced by recent innovations in diode laser design and integrated optics.
Ballistic imaging is a path-sensitive coherent imaging approach designed to overcome the negative effects of turbidity on traditional line-of-sight methodologies by exploiting the influence of scattering inhomogeneities on the temporal dynamics of photon diffusion. An ultrashort light pulse travelling through a discrete random medium is temporally dispersed into two components each characterized by uniquely different optical signatures. The first (ballistic) component is strongly attenuated and comprised of those minimally scattered transmitted photons which result from coherent interference of light scattered in the forward direction. These so-called ballistic photons propagate essentially undeviated from the incident straight-line trajectory and carry the least distorted image information and the highest degree of spatial localization regarding optical absorption. The second component encompasses the majority of re-emitted photons and results from incoherent multiple scattering associated with a diffusive random walk process through the intervening medium, and it subsequently appears as off-axis background noise (blur) in the image plane. The time-gated ballistic imaging approach seeks to isolate the early-arriving ballistic image-bearing component from the adverse effects of diffuse scattering on image formation using an ultrafast optical gate, typically less than 50 picoseconds in duration, superimposed on the transmitted photon time-of-flight distribution. Temporal gating rejects or filters out the much larger number of late-arriving photon trajectories resulting from the incoherent diffuse scattering distribution through the medium, thereby eliminating optical pathlength uncertainty over the gate interval.
Pictorially, the signal intensity measured over the gate width (integration time) which emerges from the sample depends on the inherent absorption and scattering properties within an approximately ellipsoidal volume oriented along the transillumination axis. The spatial extent of the ellipsoid is constrained by the volume of all photon paths which are possible in a given time-gated interval, and the size of each traversal section or time slice .+-..delta..tau.=n.delta.L/c of the volume represents the resulting spatial resolution of the image.
Numerous experimental methods have been demonstrated to implement the requisite temporal discrimination, including optical shutters based on transient nonlinear Kerr, photorefractive, or stimulated Raman interactions, "light in flight" interferometric and cross-correlation heterodyne/homodyne gating based on field coherence properties, electronically gated streak camera imaging, and time-correlated single photon counting. Ballistic imaging techniques such as these can achieve close to diffraction-limited resolution but suffer from extremely small signal levels due to the selective nature of the time-gating process which discards the bulk of the illumination energy to achieve maximum contrast. Moreover, since the intensity of the time-gated coherent component is attenuated approximately exponentially with penetration depth and scattering mean-free-path, general applicability is fundamentally restricted in thick scattering media by operational compromises between measurement sensitivity (minimum detectable signal), dosimetry limitations (damage threshold), and image contrast (gate width) criteria.
The ability to image through dense scattering medium is fundamentally limited by the sensitivity to small ballistic and quasi-ballistic signals, as the intensity of the coherent component is attenuated approximately exponentially with thickness and scattering mean free path. Achievable image quality is a subjective compromise between minimum detectable signal and the relative size of the diffuse component. The gate should provide substantial discrimination against the time-delayed diffusively scattered light which contains the bulk of the pulse energy, while maintaining high transmission efficiency of the image-bearing component for maximum dynamic range. Shorter gate durations imply better image contrast and improved spatial resolution, but at the expense of fewer integrated photons for signal processing. An optimum integration time has to be determined between a blurred dc image (integration time too long) and noisy image (integration time too short). The minimum detectable transmission level for ballistic imaging is fundamentally limited by the quantum shot noise of the detector, and the maximum permissible number of input photons impinging on the tissue sample is limited by dosimetry constraints determined by the ANSI exposure standards and FDA ocular safety guidelines.
Based on these considerations and typical tissue parameters, a transillumination imaging system for biological tissue should ideally be capable of detecting spatially-resolved images over a dynamic range of transmitted signal exceeding twelve orders of magnitude. The laser source should generate low-noise stable optical pulses at kilohertz or higher repetition rates and deliver .about.25 mW average powers to achieve illumination fluences adequate for image contrast over a range of penetration depths (centimeters) while maintaining sample exposure levels below the burn standard for living tissue. The source should possess wavelength versatility for spectroscopic imaging, and produce the pulse durations necessary to achieve millimeter spatial resolution with sufficient power for nonlinear gating of full 2-D images. Fast sampling achievable with mode-locked pulse trains combined with a noiseless amplifying gate could partially compensate for the extremely low number of photons integrated over a short gating periods by improving image processing rates, and minimizing the effect of dark noise statistical fluctuations on photon counting precision and radiometric fidelity of photodetector arrays. Image processing speed is essential for medical diagnosis under conditions where the measurement must be performed rapidly compared to movement of the subject (i.e., motion-induced artifacts due to breathing).
Ballistic imaging has been mentioned in a few U.S. patents prior to this invention. U.S. Pat. No. 5,371,368 of Alfano et al discloses a system for ballistic imaging an object in or behind a highly scattering medium which utilizes a large laser and a Kerr gate. U.S. Pat. No. 5,441,054 of Tsuchiya discloses a detection system utilizing phase modulation, but no amplification, of the detected signal. This patent notes that ballistic imaging is difficult due to weak signals. U.S. Pat. No. 5,528,365 of Gonatas compares the spatial distribution of photons scattered by an object to the calculated flux for a homogeneous object to yield the optical structure of the object convolved with a computed probability weighting function, from which a tomographic map of the object can be obtained. None of these patents teaches the novel use of a parametric amplifier in a ballistic imaging application.